In conventional Public Switch Telephone Networks (PSTN) and Integrated Services Digital Networks (ISDN), for example, up till now the overwhelming majority of residential and office subscribers have a wired connection from the subscriber premises to a public local exchange. These subscriber connections, which may run via an intermediate concentrator, are referred to as the local loop.
The installation of such wired links all the way to the subscriber premises is both time consuming and involves substantial networking costs, apart from troubles caused by the interruption of streets and pavements. Therefore, there is a growing interest in replacing the wired local loop by a so-called wireless local loop, i.e. using radio technology as an alternative for or as a replacement of the copper wires down to the subscriber premises. This is not only of interest for the present network operators in case of expansion or renovation of the existing public telecommunication network but, in particular, for new operators which would like to provide competitive public telecommunication services, such as a public telephone service. The concept of wireless public subscriber connections is called Radio in the Local Loop (RLL).
Within the concept of RLL, two basic systems can be distinguished: Fixed RLL (FRLL) and Mobile RLL (MRLL). In the FRLL system, the subscriber is provided with an ordinary telephone socket, however connected to a radio transceiver, also called Fixed Access Unit (FAU) or Wireless Fixed Access Unit (WFAU). Via this FAU/WFAU a radio link is established with a so-called radio access unit, which provides access to the PSTN/ISDN. In the MRLL concept, the subscriber is provided with a portable cordless or mobile radio telephone handset, by which, via the radio access unit, direct access to the PSTN/ISDN can be established.
Mixed concepts are also possible, i.e. FRLL providing mobility in the subscriber premises, also called Cordless In The Home (CITH) and residential or neighbourhood mobility, also called Cordless In The Neighbourhood (CITN). In some countries, governmental regulations prevent the established telephone operators from offering local mobility in the present PSTN/ISDN. In such cases, it is very advantageous for a second or third operator to offer both fixed and mobile or cordless access to the PSTN/ISDN.
Other types of communication systems having wired connections to a PSTN/ISDN are the well-known cellular mobile telephone and data transmission systems. A typical cellular mobile communication system comprises mobile radio subscriber units, a plurality of radio base stations, each providing service to a geographical area or cell, and radio exchanges or mobile telephone switching offices (MTSO) to which a number of base stations are connected. The MTSO are in turn coupled to a PSTN/ISDN for completing transmissions, such as telephone calls, between mobile radio subscribers and landline subscribers.
Cellular systems provide coverage over relatively wide areas, i.e. relatively large cells. Analogue cellular systems, such as designated AMPS, ETACS, NMT-450 and NMT-900 have been deployed throughout the world. Digital cellular systems are designated IS-54B in North America and the pan-European GSM system. These systems, and others, are described, for example, in the book titled "Cellular Radio Systems", by Balston et al., published by Artech House, Norwood, Mass., 1993.
First generation cellular mobile networks provide service to macrocells, having a range of 1 to 5 km from the base station to the cell boundary, and large cells (5 to 35 km), with some satellite cells (&gt;500 km). An important problem in wireless cellular communication is to provide full coverage cost effectively. This has lead to the splitting of cells in dense traffic areas, adding microcells (10 to 400 m for pedestrians and 300 m to 2 km for vehicles) and minicells (500 m to 3 km) overlaid by a macrocell structure. The overlaying macrocells serve low-traffic areas and address cell crossings by mobile subscribers.
Future cellular mobile networks will also have picocells (a few meters) and nanocells (up to 10 m), often in clusters of street microcells, with each cluster overlaid by a macrocell. In a typical cell overlay configuration, each microcell has its own base station providing service to the corresponding cell, whereas the several base stations are wired to a concentrator or access unit which is in turn coupled to a MTSO. In particular in a picocell and nanocell environment these wired links or loops involve substantial networking costs, not contributing to the object of providing cost effective cellular mobile coverage.
Accordingly, in such cellular overlay mobile networks it would be advantageous to replace the wired links between the several base stations of micro-, nano- and picocells and the corresponding access unit by wireless links, hereinafter referred to as wireless multicell links (WML).
The advantages of wireless local loop connections in the PSTN/ISDN and wireless multicell links in a cellular mobile network are numerous, ranging from short installation times, increased flexibility and improved operation and maintenance of the network to the opportunity of providing local subscriber mobility to the PSTN/ISDN. international Patent Application WO 94/19877 discloses a RLL system based on the existing business cordless technology, such as designated CT2, CT3 and Digital Eurocean Cordless Telecommunications (DECT), now called Digital Enhanced Cordless Telecommunications (DECT).
These very low power, high communication capacity systems consist of a plurality of geographically spread radio access units or base stations. Each radio access unit comprises a radio access module having radio transceiver means and antenna means providing a plurality of radio communication channels to remote telecommunication units in a cell. The various radio access units are connected to a radio exchange, which is in turn coupled to a private or public telecommunication network. Each radio access module is further provided with control means arranged to co-operate with the remote telecommunication units for adaptively selecting a free communication channel which, when occupied, is individual to a radio link connection between the radio access unit and a particular remote telecommunication unit. This type of access technique is known as Dynamic Channel Allocation (DCA).
These cordless communication systems have been developed for use in pico-, nano- and microcell applications. In order to cover extensive residential or metropolitan areas, a very large number of access units or base stations have to be installed and maintained. The building of an infrastructure for the installation and interconnection of such a large number of access units can be rather costly.
A paper by I.Brodie, titled "Performance of Dynamic Channel Assignment Techniques in a Cellular Environment", 1992 IEEE International Conference on Selected Topics in Wireless Communication", Jun. 25-26 1992 Vancouver, B.C., Canada, discloses a radio access unit or base station for establishing radio link connection with a plurality of geographically spread remote telecommunication units in a cell of a cellular mobile radio communication system. The radio access unit comprises transceiver and control means and antenna means for establishing radio link connections in accordance with the DCA type of access technique described above. The transceiver and control means and antenna means are arranged to transmit in directionally separate geographical transmission sectors and are positioned such that a resulting omnidirectional (360.degree.) coverage is obtained.
This known access unit is arranged such that a plurality of common radio communication channels is available for each of the sectors. However, as described by Brodie, no radio communication channel may be used twice on the same radio access unit even in different sectors. Accordingly, the maximum number of radio communication channels available for radio communication in a cell is limited to the plurality of common radio communication channels available to the radio access unit or base station of the cell.
It will be understood that a large amount of control equipment and processing is needed in order to operate a RLL system or WML in a pico-, nano- or microcell overlay mobile network with an access unit such as disclosed by Brodie. In particular if more than one access unit provides coverage to a particular sector. Further, not all of the potential communication channels of the system can be made available at each access unit and transmission sector, with the result that the overall traffic handling capacity is not as efficient as required to provide a viable RLL system or WML in a cellular mobile network system for use in residential and metropolitan areas.
An essential requirement for RLL and WML cellular mobile network systems is, however, enabling installations which are economic as to capacity and power. That is to say, the various components of the system have to be designed such that an optimum between geographical coverage, range, communication capacity and installation costs can be achieved, in order to provide competitive wireless connections.